Your breasts are pretty low-maintenance, and you can keep them healthy and sexy just by leaving them alone. Still, there's one move that will mess them up: repeat dieting, which causes your boobs to sag prematurely. Here, how and why plus the right way to keep your set gorgeous and firm.

How Sag Strikes

Some sag is inevitable. Gravity, breast growth spurts in your teens and early 20s, pregnancy, and breast-feeding all cause your boobs to change shape. Any change in the shape of your breasts stretches your skin's collagen and elastin, two components that make skin firm. This leaves your twins less perky over time, says Jill Weinstein, a dermatologist at Northwestern Memorial Hospital, in Illinois.

But you'll stretch out the collagen and elastin even more by constantly dieting. In fact, cycling back and forth between the same 10 pounds in your 20s, for example, can lead to premature sag by the time you hit 30. "If you keep stretching and shrinking something, it will wear out, like a sweater," says NYC nutritionist Stephanie Middleberg, RD.

The faster you gain and lose the weight via a crash diet, the worse the sag will be, because elastin and collagen endure more stress when they break down rapidly, says Mary Jane Minkin, MD, clinical professor of obstetrics and gynecology at Yale University School of Medicine. Another way crash-dieting sends your boobs south: Weight that's lost in a short period of time is almost impossible to keep off. You'll add it back, try to diet it away again...and cause more droopage.

Why Your Boobs Are Hit Hardest

Breasts are very sensitive to weight changes. "When you gain weight, fat tends to go to your female parts first, like your breasts," Dr. Minkin says. "They're also one of the first places most women lose weight from when they diet." The thinner and heavier you get, the thinner and heavier your boobs will be, and the more droopy you'll be.

Lose Weight Without Sagging

Here's how to diet the right way so you keep sag at bay. First, forgo crash diets, i.e., any plan that cuts your intake to less than 1,200 calories a day and/or promises more than 1 to 2 lost pounds a week, says Los Angeles nutritionist Natalie Rosenstock, RD. Losing more pounds than this per week breaks down more collagen and elastin.

Stick to tactics like piling more on your plate — fruits, veggies, and lean meats, that is — since consuming healthy grub keeps you from bingeing later. And don't skip meals. It slows your metabolism and makes you hungry so you gain back any weight lost.

A final benefit of the slow-and-steady route: The longer it takes to lose weight, the more likely you are to keep it off — which halts the cycle.